This invention relates to the determination of selected characteristics of signals and, more particularly, to an adaptive technique for determining a characteristic, such as frequency, of a digitally encoded version of a signal.
Of the various techniques for conveying information via electrical signals, some type of frequency control whether as a variable or a parameter is probably the most often used. In telecommunications, for example, tones of preassigned frequencies are used for multifrequency (MF) signaling, TOUCH-TONE dialing, numerous test functions, and also data transmission. Both analog filters and digital filters of numerous types have been devised for these purposes. However, the cost and complexity of available arrangements have left something to be desired given the number and variety of applications in the telecommunications industry alone. A further consideration is the presence of quantities of noise in various forms that accompany the signals in a transmission environment.
Heretofore applications of digital filtering techniques have evolved, by and large, as direct replacements of analog arrangements or by using a transformation process, such as the discrete Fourier transform (DFT) or the fast Fourier transform (FFT). The direct replacement process is an utter disregard of the inherent advantages of digital filtering processes. The transformation process, although theoretically appealing, is expensive since it generally requires complex and dedicated hardware to provide the speed necessary for real time applications.
Recent efforts utilized to provide the bandwidth saving of transmission media, such as digital speech interpolation systems, for example, place further demands on the rapid and accurate classification of digital signals into speech, data, or tones each with as many other details as are appropriate for the purpose of the classification. The determination of frequency or frequencies of a signal is an important branch of the signal classification tree. It would be highly desirable to have a fundamental and versatile approach for these sundry applications suitable for implementation in the familiar forms of the finite impulse response (FIR) and the infinite impulse response (IIR) filter.
In a particular application, the arrangement and method should proceed when a satisfactory determination is made of signal presence. Then, the process of adapting should proceed in an orderly controlled fashion using prescribed relationships between the input signal and signals being developed. The process should also be stopped when a loss of signal occurs. The process should, furthermore, take into account a number of possible occurrences before reliable frequency measurements are obtained.
An object of the invention is to provide a flexible and direct technique for frequency determination suitable for low cost implementation.
A related object of the invention is to provide digital filter implementations of high computational efficiency wherein a minimum number of multipliers are required for hardware implementations and wherein reduced calculation time is achieved for software implementations.
An object of the invention is to provide a procedural method of practicing the inventive principles taking into account the numerous situations which may prevent reliable convergence and precise frequency determination.
Another object is to provide a frequency determination procedure capable of performing reliably in the presence of interfering tones and noise.